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specimen size effect represents a potentially dangerous situation when using fracture toughness data from small
specimens to design a structure requiring larger material
thickness than the specimen size tested. In the transition
regime, a crack in a small specimen may exhibit stable ductile tearing, while a large specimen could fail by brittle fracture and have much lower toughness properties than
predicted by the small-scale test as shown in Fig. 4. In addition, tests with unstable crack growth, such as a pop-in
event, tend to be most sensitive to specimen thickness.
Most steels used in heavy structural applications have
section thicknesses insufficient to maintain plane strain
conditions at test temperatures corresponding to service
conditions. Test specifications recommend the test specimen to be the full thickness of the material to be used in
service, less the minimum amount of machining to produce the specimen. By testing specimen geometry near full
thickness, the measured toughness is representative of inservice conditions.
Weld and HAZ effects
In addition to the factors discussed above that may influence fracture toughness as measured in laboratory test
specimens, microstructural factors must be considered
when performing toughness tests on the weld HAZs of ferritic materials. Fabrication of welds in steel can lead to the
development of small, localized, and scattered regions of
low toughness in the HAZ called local brittle zones (LBZs).
These are discrete microstructural regions in the HAZ that
exhibit lower resistance to fracture initiation than the surrounding HAZ or weld material. LBZs can often produce
fracture initiation under near linear elastic conditions during fracture toughness testing. For example, in terms of
CTOD, fracture toughness can be of the order of 0.01-0.03
mm in the LBZ when the adjacent material can have a
Pass 1
Pass 2
Eliminated
HAZ
Altered HAZ
Pass 1
Unaltered
HAZ
Pass 2
A3 isotherm
A1 isotherm
Intercritically reheated
CGHAZ (IRCG)
Subcritically reheated
CGHAZ (SRCG)
Unaltered CGHAZ,
Pass 1
Note: Shaded region
indicates the etched HAZ.
Unaltered CGHAZ, Pass 2
Unaltered FGHAZ, Pass 2
Unaltered ICHAZ, Pass 2
Unaltered SCHAZ, Pass 2
Fig. 6 — HAZ microstructural regions in a multi-pass weld[12].
16
ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013
toughness of greater than 0.4 mm at the same test temperature. These effects are typically more pronounced at test
temperatures in the ductile-to-brittle transition regime and
in situations where specimen size can maximize constraint
at the crack tip in test specimens.
Figure 6, taken from API RP-2Z[12], illustrates the HAZ
microstructure generated during multipass welding with
conventional arc welding processes such as submerged arc
welding, flux-cored arc welding, or gas metal arc welding.
LBZs are often found in the intercritically and subcritically reheated HAZ zones and are the result of reheating
a zone of unaltered, coarse-grained HAZ with subsequent
weld passes. Note that the LBZs are located very close to
the weld fusion line, typically within 0.5 mm, although this
depends on the welding heat input level and other welding process parameters. LBZs will not be encountered
when testing other HAZ regions or when performing base
material fracture toughness tests.
Theoretical and experimental work demonstrates that
the likelihood of an LBZ to promote low toughness is related to LBZ length present along the test specimen crack
tip. Although a test specimen may contain LBZs, the fatigue crack tip may not intersect an LBZ of sufficient
length or in a position of high constraint (i.e., the LBZ
might be located near one of the specimen's free edges
where low constraint will occur) and consequently have little influence on the resulting measured toughness value.
In addition, if the material adjacent to and ahead of the
LBZ has sufficient fracture toughness to arrest a brittle
fracture initiating from an LBZ, the risk of brittle fracture
in the larger structure will be measurably reduced.
Specimen aspect ratio effect
Another factor that may influence measured toughness
values is the specimen geometry in terms of cross-section
dimensions. The difference between (B2B) and (BB)
SENB specimen geometries has less effect on the measured fracture toughness than specimen thickness or a/W
effects. Here, B refers to material thickness. Numerous
studies have assessed the effect of specimen aspect ratio
and most of these investigations consider the SENB geometry. A summary of some of those studies follows.
Dawes[9] reported that distinguishing between test results from (B2B) and (BB) geometries in the transition
regime when using an a/W ratio of 0.5 is difficult. Sorem[13]
reported increased scatter in CTOD results using the
(BB) specimen geometry compared to (B2B) specimen
geometry. Sorem[13] also indicated that the lower bound
CTOD results from both (B2B) and (BB) geometries
were similar if a sufficient number of samples were tested.
However, the average CTOD result was higher for the
(BB) geometry compared to (B2B) geometry.
The effect of the specimen aspect ratio on the toughness properties of weld regions in the transition regime
was reported by Machida[14]. The (B2B) geometry produced lower toughness results compared to the (BB)
specimen geometry in the transition regime. Machida determined that the lower test results from the (B2B) geom-